A purification apparatus using a monolithic catalyst is conventionally known as an apparatus for purifying the exhaust gas of an automobile. The monolithic catalyst includes a substantially cylindrical honeycomb substrate having a large number of parallel channels for permitting a gas to flow in one direction, and slurry containing a catalyst material is coated on the inner surfaces of the channels of the honeycomb substrate. When the exhaust gas flows through the channels of the monolithic catalyst in the axial direction of the honeycomb substrate, chemical reaction takes place between the exhaust gas and the catalyst material, and the exhaust gas is purified thereby.
When the exhaust gas flows through the cylindrical monolithic catalyst, the exhaust gas flowing through the circumferential channels does not pass as smoothly as the exhaust gas flowing through the central channels as viewed in the radial direction of the monolithic catalyst. For this reason, the exhaust gas purification effect is lower in the circumferential portions of the monolithic catalyst than in the central portion thereof.
In an effort to solve this problem, various measures are taken to improve the exhaust gas purification effect at the time of manufacturing the catalyst, such as coating a larger amount of slurry in the central portion of the honeycomb substrate than in the circumferential portions thereof. For example, in the coating apparatus disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2009-136833, a resistive member such as a net member or a plate member is arranged on the slurry supply side of the honeycomb substrate in such a manner that the slurry flow is decelerated in the circumferential portions and consequently the slurry is provided more in the central portion than in the circumferential portions.